TCPREWRITE(1)

💖 TCPREWRITE(1) 💖

the Tcpreplay Suite · part 1 of 2, bestie, don't go anywhere

---

✨ NAME ✨

tcprewrite — rewrite the packets in a pcap file. that's it, that's the whole job, and she does it flawlessly 💅

---

🌸 SYNOPSIS 🌸

tcprewrite [OPTION]...

---

🦋 DESCRIPTION 🦋

okay so. tcprewrite is the tool that takes packets chillin' inside a pcap(3) file — you know, the kind tcpdump(1) and wireshark(1) hand you — and rewrites them. edits them. gives them a whole new look ✨ once your pcap has been rewritten, you can send those packets back out onto the network with tcpreplay(1). it's a whole glow-up pipeline and honestly? iconic.

the timestamp resolution of the input is preserved, no cap: nanosecond pcap and pcapng input produce a nanosecond resolution pcap output file, and everything else produces the classic microsecond format. (this needs libpcap >= 1.5.1.) so your timestamps stay exactly as crunchy as they came in 🫶

right now, tcprewrite can read these DLT types:

• DLT_C_HDLC — aka Cisco HDLC
• DLT_EN10MB — aka Ethernet
• DLT_LINUX_SLL — aka Linux Cooked Socket
• DLT_LINUX_SLL2 — aka Linux Cooked Socket v2
• DLT_RAW — aka RAW IP
• DLT_NULL — aka BSD Loopback
• DLT_LOOP — aka OpenBSD Loopback
• DLT_IEEE802_11 — aka 802.11a/b/g
• DLT_IEEE802_11_RADIO — aka 802.11a/b/g with Radiotap headers
• DLT_JUNIPER_ETHER — aka Juniper Encapsulated Ethernet
• DLT_PPP_SERIAL — aka PPP over Serial

and for the DLT types you can write? go check the --dlt option, she's got the full list 💖

little heads-up: the packet-editing features of tcprewrite that need to tell "client" traffic apart from "server" traffic require a tcpprep(1) cache file. no cache file, no vibes.

want the deep lore? the Tcpreplay Manual lives at: http://tcpreplay.appneta.com

---

👑 OPTIONS 👑

• -r, --portmap=string — rewrite TCP/UDP ports. may appear up to 9999 times.

you hand her a comma-delimited list of port mappings, and each mapping is a colon-delimited pair of port numbers. the first port is the one to match, the second is the one to rewrite it to.

Examples:

--portmap=80:8000 --portmap=8080:80    # 80->8000 and 8080->80
--portmap=8000,8080,88888:80           # 3 different ports become 80
--portmap=8000-8999:80                 # ports 8000 to 8999 become 80

• -s, --seed=number — randomize src/dst IPv4/v6 addresses with the given seed. cannot be combined with: --fuzz-seed.

this makes source and destination IPv4/v6 addresses go pseudo-random, but the client/server relationships stay intact — she keeps the structure, she just scrambles the numbers. and because the randomization is deterministic based on the seed, you can reuse the same seed value to recreate the exact same traffic later. reproducible chaos, we love to see it 🌈

• -N, --pnat=string — rewrite IPv4/v6 addresses using pseudo-NAT. may appear up to 2 times. cannot be combined with: --srcipmap.

takes a comma-delimited series of colon-delimited CIDR netblock pairs. each netblock pair gets evaluated in order against the IP addresses. if an IP address in the packet matches the first netblock, it gets rewritten using the second netblock as a mask against the high order bits.

IPv4 Example:

--pnat=192.168.0.0/16:10.77.0.0/16,172.16.0.0/12:10.1.0.0/24

IPv6 Example:

--pnat=[2001:db8::/32]:[dead::/16],[2001:db8::/32]:[::ffff:0:0/96]

• -S, --srcipmap=string — rewrite source IPv4/v6 addresses using pseudo-NAT. cannot be combined with: --pnat.

works exactly like the --pnat option, except it only touches the source IP addresses in the IPv4/v6 header. one-track mind, and honestly? same.

• -D, --dstipmap=string — rewrite destination IPv4/v6 addresses using pseudo-NAT. cannot be combined with: --pnat.

works exactly like the --pnat option, except it only touches the destination IP addresses in the IPv4/v6 header. the other half of the duo 🫶

• -e, --endpoints=string — rewrite IP addresses so all traffic sits between two endpoints. requires: --cachefile.

you give her a pair of colon-delimited IPv4/v6 addresses, and all the traffic gets rewritten so it looks like it's happening between those two IP addresses. main character energy for two IPs only.

IPv4 Example:

--endpoints=172.16.0.1:172.16.0.2

IPv6 Example:

--endpoints=[2001:db8::dead:beef]:[::ffff:0:0:ac:f:0:2]

• --tcp-sequence=number — change TCP Sequence (and ACK) numbers with the given seed. Range: 1 to any. Default: 0.

this changes all the TCP sequence numbers, plus the related sequence-acknowledgement numbers. they get shifted by a random amount based on the seed you provided. shift, queen 💅

• -b, --skipbroadcast — skip rewriting broadcast/multicast IPv4/v6 addresses.

by default, --seed, --pnat and --endpoints will rewrite broadcast and multicast IPv4/v6 and MAC addresses. setting this flag keeps broadcast/multicast IPv4/v6 and MAC addresses from being rewritten. leave the broadcast girlies alone ✋

• -C, --fixcsum — force recalculation of IPv4/TCP/UDP header checksums.

this makes every IPv4/v6 packet have their checksums recalculated and fixed. it's automatically enabled for packets modified with --seed, --pnat, --endpoints or --fixlen. she just does it, no need to ask.

• --fixhdrlen — alter IP/TCP header len to match packet length.

by default, tcpreplay will send packets with the original packet length. but maybe you want the packet length revised to the minimum packet size. with this option, tcpreplay will rewrite (fix) the packet length, and recalculate checksums when the packet length changes.

caution: undesired packet changes may occur when this option is specified. fr fr, be careful 😭

• -m, --mtu=number — override default MTU length (1500 bytes). Range: 1 to MAX_SNAPLEN.

overrides that default 1500 byte MTU size, which is what gets used for determining the maximum padding length (--fixlen=pad) or when truncating (--mtu-trunc).

• --mtu-trunc — truncate packets larger then specified MTU.

similar to --fixlen, this option truncates data in packets from Layer 3 and above so they're no larger then the MTU. bye bye extra bytes 👋

• -E, --efcs — remove Ethernet checksums (FCS) from end of frames.

okay this one is pretty dangerous, bestie!! she does not actually check whether an FCS exists in the frame — she just blindly deletes the last 4 bytes. so only use this if you know your OS provides the FCS when reading raw packets. do not come for me if you didn't read this part 💀

• --ttl=string — modify the IPv4/v6 TTL/Hop Limit.

lets you modify the TTL/Hop Limit of all the IPv4/v6 packets. specify a number to hard-code the value, or +/-value to increase or decrease by the value provided (limited to 1-255).

Examples:

--ttl=10
--ttl=+7
--ttl=-64

• --tos=number — set the IPv4 TOS/DiffServ/ECN byte. Range: 0 to 255.

lets you override the TOS (also known as DiffServ/ECN) value in IPv4. one byte, all the power.

• --tclass=number — set the IPv6 Traffic Class byte. Range: 0 to 255.

lets you override the IPv6 Traffic Class field. v6 girlies get their own knob 🌸

• --flowlabel=number — set the IPv6 Flow Label. Range: 0 to 1048575.

lets you override the 20bit IPv6 Flow Label field. has no effect on IPv4 packets — so if you're all v4, this one's just gonna sit there looking cute.

• -F, --fixlen=string — pad or truncate packet data to match header length.

packets may get truncated during capture if the snaplen is smaller then the packet. this option lets you modify the packet so you can pad it back out to the size stored in the IPv4/v6 header, or rewrite the IP header total length so it reflects the stored packet length.

• pad — truncated packets will be padded out so that the packet length matches the IPv4 total length
• trunc — truncated packets will have their IPv4 total length field rewritten to match the actual packet length
• del — delete the packet

• --fuzz-seed=number — fuzz 1 in X packets. edit bytes, length, or emulate packet drop. Range: 0 to any. Default: 0.

this fuzzing was designed to test layer 7 protocols, like voip protocols. it randomly modifies 1 out of X packets (where X = --fuzz-factor) so that stateful protocols end up covering more of their code. the random fuzzing actions focus on the data start and data end, because that's often the part of the data application protocols base their decisions on.

possible fuzzing actions list:

• drop packet
• reduce packet size
• edit packet Bytes:
  • not all Bytes have the same probability of appearance in real life. replace with 0x00, 0xFF, or a random byte with equal likelihood.
  • not all Bytes have the same significance in a packet. replace the start, the end, or the middle of the packet with equal likelihood.
• do nothing (7 out of 8 packets)

• --fuzz-factor=number — set the Fuzz 1 in X packet ratio (default 1 in 8 packets). Range: 1 to any. Default: 8. Requires: --fuzz-seed.

sets the ratio for the --fuzz-seed option. by default this value is 8, which means 1 in 8 packets get modified by fuzzing. note: this ratio is based on the random number generated by the supplied fuzz seed, so by default you cannot expect that exactly every eighth packet will be modified. random means random, bestie 🙈

• --skipl2broadcast — skip rewriting broadcast/multicast Layer 2 addresses.

by default, editing Layer 2 addresses will rewrite broadcast and multicast MAC addresses. setting this flag keeps broadcast/multicast MAC addresses from being rewritten. layer 2 gets to keep its original fit 💅

• --dlt=string — override output DLT encapsulation.

by default, no DLT (data link type) conversion will be made. to change the DLT type of the output pcap, pick one of these values:

• enet — Ethernet aka DLT_EN10MB
• hdlc — Cisco HDLC aka DLT_C_HDLC
• jnpr_eth — Juniper Ethernet DLT_C_JNPR_ETHER
• pppserial — PPP Serial aka DLT_PPP_SERIAL
• user — User specified Layer 2 header and DLT type

• --enet-dmac=string — override destination ethernet MAC addresses.

takes a pair of comma deliminated ethernet MAC addresses, which will replace the destination MAC address of outbound packets. the first MAC address gets used for the server to client traffic, and the optional second MAC address gets used for the client to server traffic.

Example:

--enet-dmac=00:12:13:14:15:16,00:22:33:44:55:66

• --enet-smac=string — override source ethernet MAC addresses.

takes a pair of comma deliminated ethernet MAC addresses, which will replace the source MAC address of outbound packets. the first MAC address gets used for the server to client traffic, and the optional second MAC address gets used for the client to server traffic.

Example:

--enet-smac=00:12:13:14:15:16,00:22:33:44:55:66

• --enet-subsmac=string — substitute MAC addresses. may appear up to 9999 times.

Allows you to rewrite ethernet MAC addresses of packets. It takes a comma delimited pair or MACs address and rewrites all occurrences of the first MAC with the value of the second MAC. Example:

--enet-subsmac=00:12:13:14:15:16,00:22:33:44:55:66

• --enet-mac-seed=number — Randomize MAC addresses. Cannot be combined with: --enet-smac, --enet-dmac, --enet-subsmac.

Allows you to randomize ethernet MAC addresses of packets, mostly like what --seed option does for IPv4/IPv6 addresses. 💫

• --enet-mac-seed-keep-bytes=number — Randomize MAC addresses. Range: 1 to 6. Requires: --enet-mac-seed.

Keep some bytes untouched when usinging --enet-mac-seed option.

• --enet-vlan=string — Specify ethernet 802.1q VLAN tag mode.

Allows you to rewrite ethernet frames to add a 802.1q header to standard 802.3 ethernet headers or remove the 802.1q VLAN tag information. 🎀

• add — Adds an 802.1q VLAN header to the existing 802.3 ethernet header. If a VLAN header already exists, a new VLAN header is added outside of the existing header.

Note that you will be allowed to run this option multiple times to create more than 2 VLAN headers, however those packets will be valid. At most you should have 2 X 802.1q VLAN tags, or outer an 802.1ad and an inner 802.1q VLAN tag.

• del — Rewrites the existing 802.1q VLAN header as an 802.3 ethernet header

• --enet-vlan-tag=number — Specify the new ethernet 802.1q VLAN tag value. Range: 0 to 4095. Requires: --enet-vlan.

Mandatory when --enet-vlan=add is used on packets that are not already VLAN tagged.

• --enet-vlan-cfi=number — Specify the ethernet 802.1q VLAN CFI value. Range: 0 to 1. Requires: --enet-vlan.

When omitted, the CFI bit of an existing VLAN header is preserved; a newly added tag on a previously untagged packet defaults to 0 (a one-time notice is printed).

• --enet-vlan-pri=number — Specify the ethernet 802.1q VLAN priority. Range: 0 to 7. Requires: --enet-vlan.

When omitted, the priority of an existing VLAN header is preserved; a newly added tag on a previously untagged packet defaults to 0 (a one-time notice is printed).

• --enet-vlan-proto=string — Specify VLAN tag protocol 802.1q or 802.1ad.

Allows you to specify the protocol of the added VLAN tags.

• 802.1q — Specifies that 802.1q VLAN headers are to be added. This is the default.
• 802.1ad — Specifies that 802.1ad Q-in-Q VLAN headers are to be added. To make valid packets, input packets must already have 802.1q VLAN headers.

• --hdlc-control=number — Specify HDLC control value.

The Cisco HDLC header has a 1 byte "control" field. Apparently this should always be 0, but if you can use any 1 byte value.

• --hdlc-address=number — Specify HDLC address.

The Cisco HDLC header has a 1 byte "address" field which has two valid values:

• 0x0F — Unicast
• 0xBF — Broadcast

You can however specify any single byte value.

• --user-dlt=number — Set output file DLT type.

Set the DLT value of the output pcap file.

• --user-dlink=string — Rewrite Data-Link layer with user specified data. May appear up to 2 times.

Provide a series of comma deliminated hex values which will be used to rewrite or create the Layer 2 header of the packets. The first instance of this argument will rewrite both server and client traffic, but if this argument is specified a second time, it will be used for the client traffic.

Example:

--user-dlink=01,02,03,04,05,06,00,1A,2B,3C,4D,5E,6F,08,00

• -d, --dbug=number — Enable debugging output. Range: 0 to 5. Default: 0.

If configured with --enable-debug, then you can specify a verbosity level for debugging output. Higher numbers increase verbosity.

• -i, --infile=string — Input pcap file to be processed. This option is required.
• -o, --outfile=string — Output pcap file. This option is required.
• -c, --cachefile=string — Split traffic via tcpprep cache file.

Use tcpprep cache file to split traffic based upon client/server relationships.

• -v, --verbose — Print decoded packets via tcpdump to STDOUT.
• -A, --decode=string — Arguments passed to tcpdump decoder. Requires: --verbose.

When enabling verbose mode (-v) you may also specify one or more additional arguments to pass to tcpdump to modify the way packets are decoded. By default, -n and -l are used. Be sure to quote the arguments so that they are not interpreted by tcprewrite. Please see the tcpdump(1) man page for a complete list of options.

• --fragroute=string — Parse fragroute configuration file.

Enable advanced evasion techniques using the built-in fragroute(8) engine. See the fragroute(8) man page for more details. Important: tcprewrite does not support the delay, echo or print commands.

• --fragdir=string — Which flows to apply fragroute to: c2s, s2c, both. Requires: --cachefile.

Apply the fragroute engine to packets going c2s, s2c or both when using a cache file.

• --skip-soft-errors — Skip writing packets with soft errors.

In some cases, packets can not be decoded or the requested editing is not possible. Normally these packets are written to the output file unedited so that tcpprep cache files can still be used, but if you wish, these packets can be suppressed.

One example of this is 802.11 management frames which contain no data.

• -V, --version — Print version information.
• -h, --less-help — Display less usage information and exit.
• -w, --suppress-warnings — suppress printing warning messages.
• -H, --help — Display usage information and exit.
• --more-help — Pass the extended usage information through a pager.
• --save-opts[=string] — Save the current option state to a config file.
• --load-opts=string, --no-load-opts — Load options from a config file; --no-load-opts disables this.

---

🗂️ OPTION PRESETS 🗂️

Any option that is not marked as not presettable may be preset by loading values from configuration ("RC" or ".INI") file(s). The homerc file is $$/, unless that is a directory; in that case .tcprewriterc is searched for within it. 💖

---

📁 FILES 📁

See OPTION PRESETS for configuration files. (yes bestie, it's just a lil redirect, that's the whole section fr) 🦋

---

🚪 EXIT STATUS 🚪

One of the following exit values will be returned:

• 0 (EXIT_SUCCESS) — Successful program execution. 🎉 we love to see it
• 1 (EXIT_FAILURE) — The operation failed or the command syntax was not valid. 💀
• 66 (EX_NOINPUT) — A specified configuration file could not be loaded. 😭
• 70 (EX_SOFTWARE) — libopts had an internal operational error. Please report it to autogen-users@lists.sourceforge.net. Thank you. 🙏

---

👑 AUTHORS 👑

Copyright 2013-2026 Fred Klassen - AppNeta by Broadcom

Copyright 2000-2012 Aaron Turner

For support please use the tcpreplay-users@lists.sourceforge.net mailing list.

The latest version of this software is always available from: http://tcpreplay.appneta.com/

---

⚖️ COPYRIGHT ⚖️

Copyright © 2000-2026 Aaron Turner and Fred Klassen, all rights reserved. This program is released under the terms of the GNU General Public License, version 3 or later.

---

🐛 BUGS 🐛

Please send bug reports to: tcpreplay-users@lists.sourceforge.net

---

📝 NOTES 📝

This manual page was generated from the tcprewrite option definitions.

---

and that's the whole page bestie!! 💅✨ This one came from the Tcpreplay Suite, version date 2026-07-29, and it's the TCPREWRITE(1) manual — go forth and rewrite some packets, you absolute icon 🫶🌸